1. Field of the Invention
This invention relates to a color signal processing device arranged to convert a carrier chrominance signal temporarily into color-difference signals and to modulate the color-difference signals back into the carrier chrominance signal after performing various color signal processing actions on the color-difference signals.
2. Description of the Related Art
A digital process is performed on a composite video signal consisting of a luminance signal and a carrier chrominance signal for noise reduction, etc., by using, for example, a field memory, etc. The process can be advantageously carried out by demodulating the luminance and chrominance signals into baseband signals.
In this instance, the composite video signal is demodulated through the following two different processes:
(1) the signal is separated into the luminance signal (hereinafter referred to as Y signal) and the carrier chrominance signal (hereinafter referred to as C signal). PA1 (2) the C signal is demodulated into a color signal.
Through the above-stated process (2), baseband color-difference signals are obtained in a digital signal form in the following manner: two different sampling clock signals which are phase-locked to the color burst signal of the carrier chrominance signal are used for directly analog-to-digital (hereinafter referred to as A/D) converting the carrier chrominance signal.
The above-stated processes are described below in further detail by taking a color TV signal of the NTSC system as an example:
The color TV signal of the NTSC system is first separated into the Y and C signals. The separated C signal is A/D converted according to a sampling clock signal of a frequency which is four times as high as the frequency of the color burst signal. At that instant, if the sampling clock signal is accurately phase-locked to the color burst phases of 0.degree., 90.degree., 180.degree. and 270.degree., sample data sampled at the timing synchronized with the phase of 180.degree. can be regarded as a color-difference signal B-Y and sample data sampled in synchronism with the phase of 270.degree. as a color-difference signal R-Y. Then, the carrier chrominance signal can be demodulated into two different color-difference signals by distributing and separating these sample data according to the clock signal of a subcarrier frequency fsc.
Further, color-difference signal components are modulated into the C signal in the following manner: sample is obtained at a sampling time synchronized with the phase of 0.degree. by inverting the polarity of the sample data obtained at the sampling time synchronized with the phase of 180.degree.. Another sample data is obtained at a sampling time synchronized with the phase of 90.degree. by inverting the polarity of the sample data obtained at the sampling time synchronized with the phase of 270.degree.. Then, the sample data thus obtained are digital-to-analog (hereinafter referred to as D/A) converted in the order of the phases of 180.degree., 270.degree., 0.degree. and 90.degree..
For example, assuming that the sample data which is obtained by A/D converting the C signal at the sampling time synchronized with the phase of 180.degree. is D180, the sampling data D0 which is obtained by inverting the polarity of the sample data obtained at the sampling time synchronized with the phase of 0.degree. can be expressed as follows: EQU D0 = -(D180 - Dce)
wherein Dce
represents the DC offset value of the C signal.
In a case where the value Dce of the above-stated formula fails to coincide with the center value of the digital data of the C signal, it indicates a phase distortion occurred in the modulated C signal. In that case, color signal deterioration results from variations taking place in hue during the processes of modulation and demodulation. This has presented a problem.
Further, in some cases, a color signal for characters, figures or a title is added to the color-difference signals after the C signal is temporarily converted into the color-difference signals. In such a case, the color represented by the color signal for the characters or the title tends to vary according to phase fluctuations taking place in the sampling clock signal used in forming the color-difference signals from the C signal or in the clock signal used in forming the C signal from the color-difference signals.
Further, the two color-difference signals which are formed in the above-stated manner are sometimes used in obtaining a still picture signal. For this purpose, one-field amount of the two color-difference signals, for example, is stored in a memory. Then, the one-field amount of color-difference signals is repeatedly read out to obtain a still image. In this instance, a skew distortion must be prevented and the carrier chrominance signal must be kept continuous. To meet these requirements, the conventional color signal processing device has necessitated the use of a complex arrangement.